Weld decay-resistant austenitic stainless steel by grain boundary engineering

This paper presents an example of grain boundary engineering (GBE) for improving intergranular-corrosion and weld-decay resistance of austenitic stainless steel. Transmission and scanning electron microscope (TEM and SEM) observations demonstrated that coincidence site lattice (CSL) boundaries possess strong resistance to intergranular precipitation and corrosion in weld decay region of a type 304 austenitic stainless steel weldment. A thermomechanical treatment for GBE was tried for improvement of intergranular corrosion resistance of the 304 austenitic stainless steel. The grain boundary character distribution (GBCD) was examined by orientation imaging microscopy (OIM). The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of CSL boundaries indicated a maximum at the small roll-reduction. The corrosion rate was much smaller in the thermomechanical-treated specimen than in the base material for long time sensitization. The optimum thermomechanical treatment introduced a high frequency of CSL boundaries and the clear discontinuity of corrosive random boundary network in the material, and resulted in the high intergranular corrosion resistance arresting the propagation of intergranular corrosion from the surface. The optimized 304 stainless steel showed an excellent resistance to weld decay during arc welding.     

Grain boundary engineering of titanium-stabilized 321 austenitic stainless steel

Grain boundary engineering (GBE) primarily aims to prevent the initiation and propagation of intergranular degradation along grain boundaries by frequent introduction of coincidence site lattice (CSL) boundaries into the grain boundary networks in materials. It has been reported that GBE is effective to prevent intergranular corrosion due to sensitization in unstabilized 304 and 316 austenitic stainless steels, but the effect of GBE on intergranular corrosion in stabilized austenitic stainless steels has not been clarified. In this study, a twin-induced GBE utilizing optimized thermomechanical processing with small pre-strain and subsequent annealing was applied to introduce very high frequencies of CSL boundaries into a titanium-stabilized 321 austenitic stainless steel. The resulting steel showed much higher resistance to intergranular corrosion after sensitization subsequent to carbon re-dissolution heat treatment during the ferric sulfate–sulfuric acid test than the as-received one. The high CSL frequency resulted in a very low percolation probability of random boundary networks in the over-threshold region and remarkable suppression of intergranular corrosion during GBE.     

304不锈钢晶间腐蚀敏感性的评定

采用共聚焦显微镜测量了经不同敏化处理的304不锈钢晶间腐蚀后的晶间裂纹深度,通过统计晶间裂纹深度值的分布建立了304不锈钢晶间腐蚀敏感性的评价方法。如果晶间裂纹深度值大于0．5μm,则可认为所测试的304不锈钢发生了晶间腐蚀现象。     

Q235钢、Cu-Cr合金铸铁和TP304H不锈钢在熔融Zn-5%Al-RE合金中的耐蚀性研究

为研究铁基材料的成分和显微组织对其耐熔蚀性能的影响,将Q235钢、Cu-Cr合金灰铸铁、TP304H不锈钢及喷丸的TP304H试样在熔融Zn-5%Al-RE合金中进行了熔蚀试验(试验温度为470℃),并进行了熔蚀速率测定和截面形貌观察(SEM)。结果表明:在试验条件下,Q235钢在熔融合金中不能形成连续致密的Al-Fe金属间化合物层;Cu-Cr合金铸铁中的石墨及珠光体对Al、Fe原子扩散起到了阻挡作用;TP304H不锈钢由于奥氏体结构的致密性,Al原子在其中的扩散速率低,同时在Al-Fe化合物层之外形成了Zn-Al-Cr-RE相,对化合物层起到一定的保护作用;TP304H不锈钢经喷丸处理后由于生成马氏体,使Al原子扩散速率提高,熔蚀速率增大。4种试验材料在熔融Zn-5%Al-RE中的耐蚀性能从高到低的排序:TP304H喷丸处理的TP304HCu-Cr合金铸铁Q235钢。     

Pitting corrosion resistance of laser surface alloyed 304 stainless steel

Abstract

The pitting corrosion resistance of AISI 304 stainless steel (SS), which was laser melted in both nitrogen and argon atmosphere, was studied using the potentiodynamic anodic polarisation method. An attempt was made to introduce nitrogen onto the surface layers by melting in nitrogen atmosphere and argon atmosphere using a continuous wave CO₂ laser as the heat source at a power output of 3·06 kW at the laser head. The pitting corrosion resistance was determined by measuring the critical pitting potential during anodic polarisation. Secondary ion mass spectrometry (SIMS) was carried out on the laser melted surface to characterise the chemical composition. It was found that the pitting corrosion resistance of 304 SS was improved when laser surface melting was carried out in argon and was improved further when melting was carried out in nitrogen atmosphere. However, the improvement in pitting corrosion resistance in laser melted material was observed only in the reabraded condition and the pitting corrosion resistance in the as melted condition was lower than for the unmelted 304 SS. The improvement in localised corrosion behaviour was attributed to the increase of nitrogen content which was incorporated onto the surface layer during laser surface melting and this was confirmed using SIMS.

MST/1530     

Grain boundary engineering for improving stress corrosion cracking of 304 stainless steel

Grain boundary engineering (GBE) via low strain tension and annealing was used to enhance the resistance to stress corrosion cracking of a 304 stainless steel. Electron backscattered diffraction (EBSD) analysis exhibited that the GBE steel had a higher fraction of low-∑ coincidence site lattice (CSL) boundaries, larger grain-clusters, longer twin boundary chains, and fewer paths of connected non-twin boundaries with a more zigzag shape. Slow strain rate tests in high-temperature water showed that the GBE steel performed better plasticity, higher tensile strength, and similar yield strength compared to conventional steel. The low fraction of random boundaries in GBE steel resulted in a lower frequency of intergranular crack initiation, and the zigzag paths of non-twin boundaries made the intergranular crack propagation more difficult.     

LASER SURFACE TREATMENTS FOR PROMOTING CORROSION RESISTANCE OF A CARBON STEEL

The effect of laser treatment and laser alloying on corrosion resistance of 1045 steel has been studied. Various ways of surface alloying have been investigated by using continuous CO₂ laser beam: i) irradiation of chromium painted surfaces; ii) irradiation of Ni and Cr-electroplated surfaces; iii) direct injection of Ni and Cr powders into the melt pool. A high and uniform level of alloying in the surface layer can be achieved in the cases of electroplated surfaces and direct powder injection. It requires, however, an appropriate choice of irradiation conditions (such as beam power, beam traverse speed, beam defocusing and degree of overlapping) that provide remelted layer of a limited and fairly uniform thickness. It has been found that in order to achieve corrosion behavior of laser treated surfaces similar to that of austenite type 304 stainless steel chromium and nickel contents in the alloyed layer are to be higher than those of 304 steel.     

Application of Self Assembled 6-aminohexanol layers for corrosion protection of 304 stainless steel surface

Grafting of 6-aminohexanol onto a 304 stainless steel substrate was performed with the assistance of polydopamine self assembly. The surface structure of the films was characterized using optical and scanning electron microscopy and X-ray energy dispersive spectroscopy confirmed the establishment of organic films. The corrosion resistance properties were characterized using the electrochemical impedance spectroscopy and potentiodynamic polarization curve measurements. Enhanced corrosion resistance performance was mainly ascribed to the compact film structure and the blocking characteristics against electron transfer of the modified 304 stainless steel substrate.     

Study of the characteristics and corrosion behavior for the Zr-based metallic glass thin film fabricated by pulse magnetron sputtering process

Two sets of source target which consist of ZrCuAlNiV and ZrTiAlNiV, respectively were selected as the target materials for preparing a series of thin film coatings on the 304 stainless steel substrate by DC pulse magnetron sputtering process. The microstructures of these as-prepared ZrCuAlNiV and ZrTiAlNiV thin films were examined by X-ray diffraction and TEM observation. In parallel, the characteristic analysis of these ZrCuAlNiV and ZrTiAlNiV alloy thin films including surface roughness, and corrosion resistance were analyzed by atomic force microscopy (AFM), and tested by electrochemical method as well as salt spray testing, respectively. The results showed that the ZrCuAlNiV thin film exhibits a typical amorphous microstructure and smooth surface with average roughness about 1 nm. The ZrCuAlNiV thin film performs similar corrosion resistance to 304 stainless steel according to the result of salt spray testing in 5% NaCl solution. Additionally, the AC impedance value of ZrCuAlNiV is 20 times than 316L stainless steel and 4 times than ZrTiAlNiV, implying that the ZrCuAlNiV thin film has better corrosion resistance than the others owing to its amorphous state.     

Improvement of Intergranular Stress Corrosion Crack Susceptibility of Austenite Stainless Steel through Grain Boundary Engineering

Intergranular stress corrosion crack susceptibility of austenite stainless steel was evaluated through threepoint bending test conducted in high temperature water. The experimental results showed that the frequent and efficient introduction of low energy coincidence site lattice boundaries through grain boundary engineering resulted in an apparent improvement of the intergranular stress corrosion crack resistance of austenite stainless steel.     

Effect of excimer laser melting on the near surface chemistry and corrosion properties of aisi 304 stainless steel

We have investigated the effect of excimer laser surface melting and resolidification on the near surface chemistry and corrosion properties of AISI 304 stainless steel. Auger electron spectroscopy sputter-depth profiles show a 30–60% enhancement of Cr at the surface. Improved passivity and an increased resistance to pitting are observed.     

Microstructure and Properties of Laser Surface Alloying 304 Stainless Steel with Si

Silicon was added to the surface of 304 stain-less steel by laser melting the sprayed preplaced Sipowders.The optical microscopy,X-ray diffractionand EPMA were employed to investigate themicrostructure and chemical composition of thelaser surface alloyed layer.The hardness of the al-loyed layer was measured by microhardness testand wear resistance was evaluated by scratch test.Corrosion resistance of laser alloyed sample wasstudied in 1N H_2SO_4+0.1N NaCl and0.5N HCl+0.5N NaCl aqueous solutions.Amicrocrystalline intermetallic compound coatingwith smooth surface and good chemicalhomogeneity without porosity and crack is ob-tained.The hardness is about 720 HV.The wear re-sistance is 2 times better than that of the substrate.The corrosion resistance of the laser alloyed sampleis much better than that of 304 stainless steelsample.     

等离子体渗氮时间对304不锈钢腐蚀行为的影响研究

为改善304不锈钢在Cl-环境中的局部腐蚀性能,采用热丝增强等离子体非平衡磁控溅射技术,对其进行离子渗氮。利用扫描电镜、X射线衍射仪及电化学工作站,对不同渗氮时间下304不锈钢渗氮层的组织、结构及耐蚀性能进行了研究,并讨论了相关的腐蚀过程及耐蚀机制。结果表明,等离子体渗氮对304不锈钢的耐蚀性能有显著影响。经2 h渗氮后,304不锈钢的腐蚀电位从-257提高至-128 mV,阻抗模值较基体提高了3个数量级。而腐蚀电流密度则从7.94×10^-7降低至5.21×10^-8A/cm²,腐蚀失重从837.65 g/m²·a^-1降至159.46 g/m2·a^-1,不锈钢的耐蚀性能得到显著提高。然而,当渗氮时间延长至4 h时,由于多余的N原子与Fe原子相结合形成Fe₃N。γ_N相与Fe₃N相构成电偶,形成腐蚀微电池,使腐蚀电位显著降低,腐蚀电流密度和腐蚀失重明显增加,从而降低了不锈钢的耐蚀性能。在本实验中,渗氮2 h的304不锈钢在含Cl^-溶液中的耐蚀性最佳。     

Corrosion of a stainless steel handrail

AISI 304 and 304L stainless steels are “workhores” grades of austenitic stainless steel frequently used in architectural applications, as well as in cookware, appliances, and numerous other applications where resistance to corrosion is required. This paper examines a corrosion failure (the appearance of rustlike stains on the surface) of a 304 stainless steel handrail that appears to have occurred as a result of contamination during the fabrication process.     

316L不锈钢表面纳米化后腐蚀性能研究

对表面纳米化和未经表面纳米化处理的316L不锈钢的样品分别进行点蚀实验和应力腐蚀对比实验,在3.5%(质量分数)NaCl水溶液中分别测出它们的极化曲线.结果表明,316L不锈钢表面纳米化后抗点蚀性能下降,抗应力腐蚀性能提高.对应力腐蚀断口的SEM 分析发现,316L不锈钢应力腐蚀断口有明显分区现象,断裂形式为韧性断裂,开裂通道既有穿晶型也有沿晶型.     

Intergranular stress corrosion cracking of acoustic-filter hooks used in car

Some acoustic-filter hooks used in car fractured in service. The failure mechanism of the hooks is intergranular stress corrosion cracking. The necessary conditions (susceptible composition of the hook materials; presence of chloride in service environment; presence of residual tensile stress and occurrence of sensitization in the fracture region) caused intergranular stress corrosion mechanism. Non-qualified chemical composition of the hooks (much higher carbon content than the specified of AISI 304 stainless steel) is mainly responsible for the failure of the hooks, which is a key inherent factor.     

16Cr奥氏体不锈钢晶间腐蚀的敏感性

为了研究1Cr17Mn6Ni5N奥氏体不锈钢(16Cr奥氏体不锈钢)的晶间腐蚀行为,通过光学显微镜(OM)、X射线衍射仪(XRD)和晶间腐蚀试验研究了其在不同敏化温度和冷却方式下,晶间碳化物的析出和耐晶间腐蚀性能的变化。结果表明:16Cr奥氏体不锈钢在敏化温度区间内加热时,晶界碳化物随加热温度的上升而增加,加热温度为850℃左右时晶界析出碳化物最多,主要为Cr_(23)C_6和Cr_7C_3;在敏化温度区间内相同加热温度时,水冷可显著减少其晶界碳化物的析出;16Cr奥氏体不锈钢对晶间腐蚀不敏感。     

316L-16MnR复合板不锈钢侧晶间腐蚀原因分析

采用GB／T4334．5—2000标准方法对不锈钢-低合金钢复合板进行晶间腐蚀试验,对不锈钢侧产生裂纹的原因进行了分析。结果表明,由于不锈钢板与低合金钢板的结合面上存在着不可抗拒的碳迁移现象,致使不锈钢板侧过渡区内的奥氏体晶界上聚集着大量的网状碳化物,从而造成晶界上的缺陷,使其在经过硫酸．硫酸铜溶液的腐蚀后,进行弯曲时产生了大量的晶间腐蚀裂纹。     

原子力显微镜评价奥氏体不锈钢晶间腐蚀敏感性的研究

由13Cr可焊马氏体不锈钢(WMSGSS)制造的油气管线由于价格便宜,相对于昂贵的耐腐蚀合金来说是一个很有竞争力的选择。但是实验室试验和现场经历表明,13Cr马氏体不锈钢焊接接头热影响区发生了沿晶应力腐蚀开裂(IGSCC)。对于低等级的马氏体不锈钢来说,沿晶应力腐蚀开裂机理被认为是在晶界的碳化物附近存在贫铬区。对于中等级和高等级马氏体不锈钢来说,只在实验室试验发现有应力腐蚀开裂。它们的应力腐蚀开裂机理还没有明确,虽然最可能的应力腐蚀开裂机理也被认为和低等级马氏体不锈钢的应力腐蚀开裂机理类似。原子力显微镜(AFM)已开始广泛用于材料显微组织和环境敏感断裂的研究中。高等级马氏体不锈钢焊接接头对晶间腐蚀/应力腐蚀开裂敏感性很有希望由原子力显微镜检测到。在原子力显微镜技术应用于这项研究之前,用由304不锈钢准备的不同敏化度的试样来确定它的适用性是十分有益的。本文用原子力显微镜研究了不同敏化程度的304不锈钢的晶间腐蚀敏感性并和SEM和EDS结果进行了对比。     

Desensitization of sensitized 304 stainless steel by laser surface melting

Laser surface melting was used to desensitize the surface layer of sensitized 304 stainless steel. The degree of sensitization was determined quantitatively for sensitized and sensitized then laser surface melted samples from the modified ASTM-262 practice E test to be 45% and 0%, respectively. Grain-boundary melting which occurs in the heat-affected zone is believed to contribute the desensitization in the solid. X-ray diffraction results did not show any phase transformation in the melted layer or in the heat-affected zone. The results of the tensile tests indicate that sensitized stainless steel regains its corrosion resistance properties and, in addition, its mechanical properties seem to be enhanced by the laser surface melting.